Generally, a thermal recording material has a substrate and a heat-sensitive recording layer that is formed thereon and that contains, as main components, a generally colorless or light-colored electron-donating dye precursor and an electron-accepting compound. When the thermal recording material is heated with a thermal head, a hot pen, a laser beam, etc., the electron-donating dye precursor and the electron-accepting compound readily react with each other to give a recorded image. Such thermal recording materials are used in broad fields of measuring recorders, facsimile machines, printers, computer terminals, labeling machines, automatic vending machines of railway tickets or other tickets, and the like, owing to advantages that recordings are made with so simple apparatus, their maintenance is easy and they make no noise. In recent years in particular, thermal recording materials have come to be used for receipts of gas rates, water rates, electric rates, etc., slips, various receipts, etc., of ATMs of banking facilities, recording papers of financial affairs, thermal recording labels for POS systems, thermal recording tags, etc.
As the use fields thereof become so diversified, there are demanded thermal recording materials that are highly sensitive and free of ground fogging and, further, that exhibit the adherence of less deposit to a thermal head and are excellent in thermal head-matching property.
In general, the electron-donating dye precursor and the electron-accepting compound as thermal recording components for a thermal recording material are often used in the form of dispersed particles. For achieving higher sensitivity, it is sufficient to use them after they are pulverized into particles that are as fine as possible. However, when they are pulverized into particles that are too fine for achieving ultrahigh sensitivity, the coloring sensitivity is improved, but the ground fogging is intensified, so that the dispersing of them has its own limit.
Therefore, there has been developed a method in which a heat-insulating intermediate layer containing a pigment is formed between the support and thermal layer of a thermal recording material in order to achieve higher sensitivity. For example, there is disclosed a method in which an oil-absorbing pigment is incorporated into an intermediate layer (for example, see JP59-155097A). Further, there are disclosed a method in which hollow particles to be obtained by expanding are incorporated into an intermediate layer (for example, see JP59-5093A), a method in which non-expandable hollow particles are incorporated into an intermediate layer (for example, see JP62-5886A) and a method in which cup-shaped particles each having an opening portion are incorporated into an intermediate layer (for example, see JP10-217608A).
When these heat-insulating intermediate layers are formed, thermal recording materials are improved in thermal response and improved in printed image quality. However, in the method in which an oil-absorbing inorganic/organic pigment is incorporated into an intermediate layer, the properties of oil absorption and heat insulation are generally materialized by the porous structure of the pigment, and yet it has porosity (high specific surface area). Therefore, when a water-soluble adhesive is used, it is required to use a large amount of the adhesive for imparting sufficient strength that an adhesive layer should have. However, such a large amount of the adhesive impairs the porosity, and as a result, the intermediate layer is degraded in the property of heat insulation, and the improvement of the thermal response is inevitably limited. On the other hand, the method in which hollow particles are incorporated into an intermediate layer is free from a decrease by an adhesive in the property of heat insulation. Since, however, the particles per se have almost no property of oil absorption, the intermediate layer cannot absorb all of deposit that occurs from the thermal layer in printing on a thermal recording layer, and there is a problem that deposit left by the printing adheres to a thermal head to degrade the printed image quality.